Vapor deposition methods or sputtering methods have been used as methods of forming a metal film. Among vapor deposition methods, vacuum evaporation methods and the like are often used in which an electron beam or high frequency is used. Among sputtering methods, for example, a technique is often used in which plasma is generated by direct-current power supply or alternating-current power supply, and then sputtering a metal serving as the anode using the plasma to deposit the metal on the cathode. However, vapor deposition methods and sputtering methods require a vacuum process, so that they have a problem of cost and have a difficulty in upsizing or mass production.
Coating methods and the like are also well known as methods of forming a metal film. Among coating methods, for example, a technique is used in which a metal paste is applied and after drying it is fired. However, such coating methods require a high temperature of 650° C. or more for firing and therefore are not necessarily satisfactory as methods of forming a metal film. Chemical vapor deposition methods and the like are also known as methods of forming a metal film. However, such methods require a high-temperature process, as with the coating methods, and do not necessarily provide sufficient adhesion as well.
On the other hand, in recent years, there have been attempts to form a patterned metal film on a substrate using a paste in which ultrafine particles are dispersed. Examples of such known methods include a method of forming an ultrafine particle electrode, in which a metal paste is prepared by dispersing composite metal ultrafine particles composed of metal cores and organic coating layers in a solvent, and then applying the metal paste onto the electrode of a semiconductor device, and firing the metal paste at low temperature, disclosed in Japanese Unexamined Patent Application Publication No. 2001-168141. However, a metal thin film using ultrafine particles of a precious metal such as Ag or Au does not easily react with a smooth inorganic oxide substrate and therefore sufficient adhesion cannot be obtained. In particular, when a metal plating film is formed on a thin metal wire to increase conductivity, the metal wire is peeled off from the inorganic oxide substrate due to pre-plating treatment or chemical treatment using a plating bath.
In recent years, methods of forming a metal film by aerosol deposition have been also investigated. Japanese Unexamined Patent Application Publication No. 2011-153329 discloses a method of forming a metal thin film serving as an electrode or wiring pattern on a substrate by aerosolizing metal particles and spraying the aerosolized metal particles on the substrate. Japanese Unexamined Patent Application Publication No. 2013-129887 discloses a method of forming a metal thin film serving as an auxiliary electrode on a transparent electrode by an aerosol deposition method. However, aerosol deposition methods make an adhesion between an electrode and a substrate very poor, and they are not suitable especially for forming a thick film serving as an electrode. Further, aerosol deposition methods are not satisfactory because they require a vacuum process during the formation of the film or require high-temperature heat treatment after forming a film.